The present invention relates to a deflection yoke to be used with a cathode ray tube of a display device and, particularly, to a deflection yoke having a canceler coil for cancelling undesired magnetic field radiation produced by the deflection yoke around the display device
It is a recent tendency to restrict undesired magnetic radiation around a display device to a level not higher than a predetermined value. Among others, in the extremely low frequency (ELF) band covering a frequency range from 5 Hz to 2 kHz and the very low frequency (VLF) band covering a frequency range from 2 kHz to 400 kHz, such undesired magnetic field comprises a portion of magnetic field generated by a deflection yoke mounted on a cathode ray tube, as a leak magnetic field. Particularly, a special procedure is necessary to reduce an amount of magnetic flux leakage in the VLF band since the frequency range covered thereby may provide adverse effect on human body.
Various methods have proposed for suppressing such leak field which use correction coils and are disclosed in, for example, IBM Technical Disclosure Bulletin, Vol. 31, No. Jun. 1, 1988, pp 119 to 122, U.S. Pat. No. 4,922,167 in which two pairs of coils are used, Japanese Kokai 2-123647 corresponding to Italian application No. 22475 A/88, and U.S. Pat. No. 5,049,847 in which two pairs of coils are used.
These prior arts disclose the use of cancelling coils on the deflection yoke for cancelling the magnetic flux leakage.
FIG. 7 shows schematically a typical example of a conventional cancelling coil system for cancelling undesired magnetic field radiation, which is disclosed in Japanese Kokai No. 2-46085 corresponding to European application No. 88305986.7
As shown by solid arrows in FIG. 7, a leakage magnetic field 7 from a deflection yoke 9 exists in front and rear regions 12 of a display device. A pair of loop coils 4 forming a cancelling coil are arranged above and below a front end portion of the deflection yoke 9, respectively, with certain angles with respect to an axis (Z) of a cathode ray tube 10. By supplying horizontal deflection currents to the loop coils 4, a magnetic field 6 opposite in direction to the undesired leakage field 7 from the deflection yoke 9 is generated by the loop coils 4 as shown by dotted lines.
With such conventional cancelling coil system for leak magnetic field in the front and rear regions of the display device, it is unavoidable that portions of a required deflection magnetic field in the vicinity of the deflection yoke are also cancelled out. In such case a, locus of an electron beam passing through the deflection field is varied thereby, causing performances of the deflection yoke to be degraded. One of the performances of the deflection yoke which are influenced by such undesired cancellation of required magnetic field is the so-called "mislanding" or "purity-offset". This is caused by deviation of the deflection center of the electron beam under influence of the cancelling magnetic field.
FIG. 8 shows a distribution of undesired leak magnetic field radiation around the display device, which is caused by the deflection yoke, in which FIG. 8(a) shows points which are set on a circle having the cathode ray tube 10 as a center in a horizontal plane (Y=0) containing the axis (Z) of the cathode ray tube 10 and radius of a half of the full length of the cathode ray tube plus 50 cm and at which magnetic field strength are measured, the points being angularly separated from each other by 22.5.degree., and FIG. 8(b) shows leak magnetic field strength measured at the respective points in polar-coordinates.
Due to the structure of the deflection yoke 9, the field strength of undesired magnetic radiation measured in a front region of the cathode ray tube 10 is larger than that measured at a rear region. Therefore, it is necessary to make an amount of cancelling magnetic field in the front region of the tube larger than that in the rear region of the tube. For this reason, the loop coils of conventional cancelling coil disclosed in such as Japanese Kokai No. 2-46085 are arranged in backwardly opened relation to each other to regulate correction ratio of the front region to the rear region.
FIGS. 9 and 10 show vector diagrams of the deflection magnetic field 7 generated by the horizontal deflection coils 2 and the cancelling magnetic field 6 generated by the cancelling coils 4 in a region in which electron beams are to be deflected, respectively, with only an upper half of the deflection yoke being illustrated in cross section taken in a plane including the axis (Z) of the cathode ray tube.
In FIG. 9, it is clear that, within and in a front and a rear regions of the deflection yoke, the deflection magnetic field 7 generated is upward generally. In FIG. 10, it is clear that the conventional, inclined loop coils 4 mounted above and below the front end portion of the deflection yoke generate a downward cancelling magnetic field in the front and the rear regions of the deflection yoke, which is opposite to the deflection field.
FIG. 11 shows distributions of the deflecting field and the cancelling field 6 shown in FIGS. 9 and 10 measured on the center axis of the deflection yoke (Z axis), with ordinates being plus for positive direction of the Y axis. In FIG. 11, the deflecting field 7 is normalized with its peak value and scaled on the right side ordinate The cancelling field 6 is indicated in absolute value on the left side ordinate. In FIG. 11, relative positions of a ferrite core of the deflection coil, the horizontal deflection coil and the cancelling coil are shown by lines 1, 2 and 4, respectively. As shown by the curve 6 in FIG. 11, the loop coils 4 generate a magnetic field in the same direction as that generated by the horizontal deflection coil 2 within the region defined by the latter. However, the direction of the cancelling field 6 is opposite thereto in the front and rear regions of the horizontal deflection coil 2. As will be clear from FIG. 11, the center of the deflection field 7 after corrected by the cancelling field 6 is deviated backwardly since the cancelling field strength in the front region is larger than that in the rear region. Such backward deviation of the deflection center causes an electron beam to misland on a screen of the cathode ray tube.
The principle of generation of the mislanding is illustrated schematically in FIG. 12. Electron beam path is changed from a path 14 to a path 13 under influence of the cancelling field and, when the deflection center 20 is shifted rearwardly by .DELTA.Z, an incident angle of the electron beam onto a shadow mask 18 is reduced, so that the electron beam strikes fluorescent material on the screen 17 of the cathode ray tube at a position shifted inwardly of the screen 17 toward the center by .DELTA.X. This is the phenonenon called mislanding. For a 14-inch cathode ray tube, when a cancelling coil having no core is used, the shift .DELTA.X becomes about 10 .mu.m or, when the cancelling coil has a magnetic core, the shift becomes about 20 .mu.m.
Due to this mislanding, the shadow mask has to be modified again.